In many instances, it is necessary to locate, position and support a work piece which is to be milled, sanded, ground, etc. The precision and stability of the positional device is important, as is the ability to re-position the positional device. Positive location of a work piece is described in U.S. patent application Ser. No. 09/108,792 now U.S. Pat. No. 6,186,567 entitled "Automatic Clamping and Placement Holder" and which describes the use of an air and vacuum driven work piece holder which uses the positioning locator of a milling machine to locate the holder. The Automatic Clamping and Placement Holder works in conjunction with a system described in U.S. Pat. No. 5,562,276 entitled "Locator and Hold Down system for a Machine" which describes an integrated system for using holders. Both of these systems are incorporated herein by reference.
Most of the equipment described in the above references is sufficient for work pieces thicker than about 0.5 inches, even where downward forces are applied. For materials which are less than about 0.5 inches and especially for material which is less than 0.25 inches, the support of the material at the situs where the grinding tool operates on the work piece is a severe problem, especially where downward force is used. In larger pieces, support by a series of round vacuum hold downs located 2-3 inches from the outer edge has worked well.
In the thicker materials, the forces produced by the grinding tool at the edge of the work piece, are easily resisted in the volume of the work piece extending between the hold down and the worked edge. However, in thinner materials, a work piece support which is spaced 1-2 inches from the edge typically has a material strength that cannot withstand a bending moment on the length of material between the edge and the support.
Given the materials cost and the average amount of work time which goes into a work piece at failure, each failure has a significant cost in both materials cost, machine time, and labor required to clean up the breakage debris and re-load the machine.
Larger supports are simply not designed for the close-in location and spacing which would be needed to provide the close-in support, at the work piece edge to drastically reduce the material failure during the manufacturing process. Another reason that the larger supports are not utilizable is that their vacuum hold-down capability is usually achieved against a surface which is soft and therefore height variable, as well as frictionally deficient. Most hold downs have isolated circular or rectangular projections to form the support for the work piece. A vacuum is introduced around a cup surrounding the upper periphery of the support. The downward force is generated by the area of the upper periphery of the support which is evacuated, and limited only by the isolated circular projections. The rubber cup provides vacuum within its periphery but it does not provide support, even at its periphery. Thus even where grinding were to occur at the outer edge of a large vacuum support, which would be unadvisable and unsafe and too close to the rubber cup, a downward bending force over a one to two inch length of the material would still exist. Further, since most major supports are round and have a diameter of from about six to eight inches, there would be little coverage available between the main supports along a straight edge.
Another problem with close support of a work piece at its edge relates to the need to exactly control the height of closely spaced supports. Where one support is slightly taller or shorter than an adjacent support, the support itself can exert a harmful shear force on the material of the work piece and actually cause it to be more prone to failure during the grinding and polishing operation.
What is therefore needed is a support specially formulated to closely support a thin work piece near its edge, eliminating the rubber cut over extension seen in main supports, and which can provide even, secure support to a work piece.